The present invention relates to high data rate optical fibers for telecommunication systems and to methods of making such fibers.
It has been known that solitons can be generated in optical fibers when the transmission power is in the nonlinear region. The optical soliton maintains its narrow temporal pulse as it propagates down the fiber because the dispersion is balanced with the nonlinear index. Mathematically this phenomenon is adequately described with the well known nonlinear Schroedinger equation. See, for example, the publication, C. Sien, "Concatenated Soliton Fibre Link", Electronics Letters, volume 12, pages 237-238 (1991). There are three important terms in the nonlinear Schroedinger equation. These terms relate to attenuation, the group velocity dispersion and the nonlinear index effects. The balancing of the group velocity dispersion with the nonlinear index term has received much attention to date and is well known. However, pulses propagating in real fibers undergo attenuation; this can cause soliton pulses to develop frequency chirping and subsequent broadening and to then become essentially linear.
As used herein the term "dispersion" means group velocity dispersion, which is the total of the material dispersion and the refractive index profile dispersion.
It has been proposed that a soliton can survive in a fiber with loss if the group velocity dispersion can be made to decrease approximately exponentially with distance (K. Tajima, "Compensation of Soliton Broadening in Nonlinear Optical Fibers with Loss", Optics Letters, volume 12(1), pp. 54-56, 1987). In this way, the group velocity dispersion is made to continuously change so that it matches the changing power level. That publication states that this can be accomplished by varying the core diameter through fiber tapering and that such a fiber can be manufactured by controlling the fiber draw speed. Such a fiber is illustrated in FIG. 1 wherein the diameter of fiber 3 exponentially decreases from the large diameter input end 4 to the small diameter output end 5. The diameter of the core of fiber 3 is proportional to the outside diameter of the fiber. In the theoretical example proposed by Tajima the effective core diameter of such a fiber changes exponentially from about 10 .mu.m to about 5 .mu.m over 100 km.
A dispersion decreasing fiber was actually made by varying the speed of the fiber draw to change the fiber outer diameter from 175 .mu.m to 115 .mu.m, whereby the measured dispersion decreased from 10 ps/nm-km to 1 ps/nm-km over a 1 km length (V. A. Bogatyrev et al., "A single-mode fiber with chromatic dispersion varying along the length", Journal of Lightwave Technology, volume 9(5), pages 561-566, 1991). Subsequently, that fiber was used to generate a continuous soliton pulse train at 70 Gb/s (S. V. Chernikov, "70 Gbit/s fibre based source of fundamental solitons at 1550 nm", Electronics Letters, volume 28(13), pages 1210-1211, 1992).
Dispersion decreasing fibers have potential application in ultrahigh bit rate telecommunication systems. Dispersion decreasing fibers having lengths of about 100 m to 10 km can be employed in pulse compression systems employed in the generation of high bit rate soliton input signals. FIG. 2 schematically illustrates a part of a soliton communication system wherein a high bit rate pulse train is input to amplifier 7 and coupled to dispersion decreasing fiber DDF-1. The dispersion decreases exponentially with length between input end a and output end b of fiber DDF-1. After propagating a distance that is limited by the maximum dispersion change, the optical signal is again amplified at amplifier 8 and coupled to a similar dispersion decreasing fiber DDF-2, which has a high dispersion end a adjacent amplifier 8 and a low dispersion end b adjacent amplifier 9. The proposed length of fiber DDF-1 and DDF-2 is about 1-100 km. Soliton transmission becomes practical at bit rates greater than 10 Gbps.
In addition to enabling the transmission of high data rates, soliton transmission can increase the length of fiber over which the signals can be transmitted without amplification. Thus, the distances between amplifiers 7 and 8 and between amplifiers 8 and 9 could be extended by employing appropriate dispersion decreasing optical fiber.
A tapered fiber, in which the outside diameter as well as the core diameter changes to the extent proposed in the Tajima and Bogatyrev et al. publications, will introduce splicing, testing and cabling problems. As the outer fiber diameter varies, the diameter of one end of the fiber would be larger than that of a standard single-mode telecommunication fiber; this could cause problems when automatic fusion splicing equipment is employed. Moreover, the large core diameter end of the fiber would have a mode field diameter larger than that of a standard fiber, thus introducing an unacceptable splice loss. The proof test operation is somewhat complicated since a constant diameter is assumed by present proof test machines. Also, the calculation of installed stress for bent fibers is complicated by the variation in cross-sectional area along the fiber length.